In the past, cans for packaging carbonated beverages, such as soft drinks or beer, have been formed from metal, typically aluminum. Such cans are conventionally made by attaching a can end, or lid, to a drawn and ironed can body that has an integrally formed bottom.
Certain parameters relating to the geometry of the can bottom play an important role in the performance of the can. In can bottoms employing an annular nose, discussed further below, the diameter of the nose affects the ability to stack or nest the bottom of one can into the top end of another can. Nose diameter also affects the resistance of the can to tipping over, such as might occur during filling.
In addition to stacking ability and anti-tipping stability, strength is also an important aspect of the performance of the can bottom. For example, since its contents are under pressure, which may be as high as 90 psi, the can must be sufficiently strong to resist excessive deformation due to internal pressurization. Therefore, an important strength parameter for the can bottom is buckle strength, which is commonly defined as the minimum value of the internal pressure required to cause reversal, or inversion, of the domed portion of the can bottom--that is, the minimum pressure at which the center portion of the can bottom flips from being concave downward to convex downward. Another important parameter is drop resistance, which is defined as the minimum height required to cause dome inversion when a can filled with water and pressurized to 60 psi is dropped onto a hard surface.
In addition to satisfying performance requirements, there is tremendous economic incentive for can makers to reduce the amount of metal used. Since billions of such cans are sold each year, even slight reductions in metal usage are desirable. The overall size and general shape of the can is specified to the can maker by the beverage industry. Consequently, can makers are constantly striving to reduce the thickness of the metal by refining the details of the can geometry to obtain a stronger structure. Only a few years ago, aluminum cans were formed from metal having a thickness of about 0.0112 inch. However, aluminum cans having thicknesses as low as 0.0108 inch are now available.
One technique for increasing the strength of the can bottom that has enjoyed considerable success is the forming of a outwardly concave dome in the can bottom. Beverage cans, such as those for soft drinks and beer, typically have a side wall diameter of about 2.6 inches. Conventionally, the radius of curvature of the bottom dome is at least 1.550 inch. For example, U.S. Pat. No. 4,685,582 (Pulciani et al.), assigned at issue to National Can Corporation, discloses a can having a side wall diameter of 2.597 inches and a dome radius of curvature of 2.120 inches. Similarly, U.S. Pat. No. 4,885,924 (Claydon et al.), assigned at issue to Metal Box plc, discloses a can having a side wall diameter of 2.59 inches and a dome radius of curvature of 2.0 inches, while U.S. Pat. No. 4,412,627 (Houghton et al.), assigned at issue to Metal Container Corp, discloses a can having a side wall diameter of 2.600 inches and a dome radius of curvature of 1.750 inches.
The strength of a domed can bottom is further increased by forming a downwardly and inwardly extending frustoconical wall on the periphery of the bottom that terminates in an annular bead, or nose. The nose has circumferentially extending inner and outer walls, which may also be frustoconical. The inner and outer walls are joined by an outwardly convex arcuate portion, which may be formed by a sector of a circle. The base of the arcuate portion forms the surface on which the can rests when in the upright orientation.
According to conventional can making technology, the radius of curvature of the inner surface of the arcuate portion of the nose in such domed, conically walled can bottoms was generally 0.050 inch or less. For example, prior to the development of the current invention, the parent of the assignee of the instant application, Crown Cork & Seal Company, sold aluminum cans with 202 ends (i.e., the diameter of the can end opposite the bottom is 22/16 inch) in which the radius of curvature of the inside surface of the nose was 0.050 inch. Similarly, U.S. Pat. No. 3,730,383 (Dunn et al.), assigned at issue to Aluminum Company of America, and U.S. Pat. No. 4,685,582 (Pulciani et al.), assigned at issue to National Can Corporation, disclose a nose having a radius of curvature of 0.040 inch.
Moreover, it was heretofore generally thought that the smaller the radius of curvature of the nose, the greater the pressure resistance of the can bottom, as discussed, for example, in the aforementioned U.S. Pat. No. 3,730,383. Consequently, U.S. Pat. No. 4,885,924 (discussed above), U.S. Pat. No. 5,069,052 (Porucznik et al.), assigned at issue to CMB Foodcan plc, and U.S. Pat. No. 5,351,852 (Trageser et al.), assigned at issue to Aluminum Company of America, all disclose methods for reducing the radius of curvature of the nose in order to increase the strength of the can bottom. U.S. Pat. No. 5,351,852 suggests reworking the nose so as to reduce its radius of curvature to 0.015 inch, while U.S. Pat. No. 5,069,052 suggests reworking the nose so as to reduce its radius of curvature on the inside surface to zero and on the outside surface to 0.040 inch or less.
In addition to its geometry, the manufacturing apparatus and techniques employed in forming the can bottom can affect its strength. For example, small surface cracks can be created in the chime area of the can bottom if the metal is stretched excessively when the nose is formed. If, as sometimes occurs, these cracks do not initially extend all the way through the metal wall, they may go undetected during inspection by the can maker. This can result in failure of the can after it has been filled and closed, which is very undesirable from the standpoint of the beverage seller or the ultimate customer. The smaller the radius of curvature of the nose, the more likely that such cracking will occur. Since the radius of curvature of the nose adjacent its inner wall is thought to have a greater impact on buckle strength than the radius adjacent the outer wall, some can manufacturers have utilized a nose shape that is more complex than a simple circle sector by employing two radii of curvature--a first inside surface radius of curvature adjacent the outer wall that is above 0.060 inch and a second inside surface radius of curvature adjacent the inner wall that is below 0.060 inch. For example, U.S. Pat. No. 4,431,112 (Yamaguchi), assigned at issue to Daiwa Can Company, discloses a domed can bottom, although one that does not have a conical peripheral wall, with a nose having a first radius of curvature adjacent its inner wall of about 0.035 inch (0.9 mm) and a second radius of curvature adjacent its outer wall of about 0.091 inch (2.3 mm). Another can manufacturer has employed a domed, conically walled bottom in a 204 end can in which the inner surface of the nose, whose outer wall is inclined at an angle of about 26.5.degree. with respect to the can axis, has a first radius of curvature adjacent the nose inner wall of about 0.054 inch and a second radius of curvature adjacent the outer wall of about 0.064 inch.
Notwithstanding the improvements heretofore achieved in the art, it would be desirable to provide a can bottom having a geometry that optimized performance, especially with respect to buckle resistance, drop resistence, and stackability and manufacturability.